1. Field of the Invention
The present invention relates to drilling fluids for use in well bores which traverse subterranean formations, and in particular to water base drilling mud compositions and weighting agents for such compositions.
2. Description of the Prior Art
In the drilling of holes for exploration and extraction of crude oil and natural gas, a high density clay-water slurry, called drilling mud, is circulated through the well bore. The drilling mud is pumped down the drill stem through the openings in the drill bit and upwardly through the annular space between the drill stem and the walls of the bore to the surface of the earth. The principal purpose for introducing the drilling mud into the bore hole is to transport the cuttings produced by the drill bit to the surface of the earth. The drilling mud also lubricates the bit and the drill stem, transfers heat from the drill bit and develops a hydrostatic pressure head to prevent flow into the well hole of formation fluid such as oil, gas and water from the various strata penetrated by the drill bit.
To provide the proper hydrostatic pressure, the specific gravity of the mud is increased by adding a weighting agent such as barite. Such weighting agents increase the density of the mud and help control subsurface pressures and support the walls of the well bore. Viscosifying clays such as bentonite are added to increase the capacity of the mud for suspending the weighting material and transporting the drill cuttings. Sodium montmorillonite (bentonite) is the most important viscosifier in fresh water muds. A water slurry of bentonite has a relatively low viscosity and is free flowing, but when agitation or circulation is halted, the slurry sets or gels. Bentonite gel formation is rapid enough to permit the cuttings to fall only a short distance before the gel structure is sufficiently strong enough to support them.
Although the mud must be sufficiently viscous to carry the cuttings from the well bore and suspend particles of the weighting agent, the mud viscosity must not be so high as to interfere with the action of the pumps which circulate the drilling fluid in the formation. Thus, in the selection of a viscosifier gel and a weighting agent for a water base mud, the resulting mud slurry must be able to seal off the strata and develop the requisite hydrostatic pressure to prevent a blow-out. To provide the proper hydrostatic pressure, the specific gravity of the mud must be increased by adding a material heavier than the gel material. On the other hand, the drilling fluid must have the proper viscosity and be thick enough to carry off the cuttings, but be thin enough to be pumped and to allow the coarse cuttings to settle out in the surface mud pits so that the mud can be recycled.
In addition to the foregoing considerations, it is desirable to minimize the solids concentration in the mud to obtain maximum bit penetration rate. Therefore, the mud is generally made up with the highest yield clay available. Bentonite is the most important of such clays and typically yields 90 to 100 bbl/ton in fresh water.
The drilling penetration rate diminishes as the number of solid particles in a drilling mud increases. The combination of barite and bentolite in water base muds minimizes the solids concentration while developing the required viscosity and density. Although both bentonite and barite are presently available in commercial quantities, barite is relatively scarce and expensive. Thus, there is a continuing interest in developing high yield clays and weighting agent substitutes for barite.